The present invention is applicable to the treatment of electrodeposited copper foil. Much of the copper foil formed by the electrodeposition process is used in the printed circuit board industry. When used for printed circuit boards, the electrodeposited copper foil is bonded to a dielectric substrate. It is generally known that bare electrodeposited copper foil exhibits poor adhesion properties because of its relatively smooth surfaces. To improve the adhesion properties, it is known surface treat the copper foil. Such surface treatment may comprise adding a layer of dendritic copper or copper oxide particles to the base copper foil. Other metals, such as brass or chromium, may also be added to the foil to enhance the physical properties thereof.
Treatment of an existing foil typically does not require the high power levels necessary in electroforming foil, and therefore, treatment line speeds are generally three to four times the speed of an electroforming apparatus. In this respect, the speed and efficiency of a foil treating line is limited not so much by power requirements, but by the ability to control certain other operating parameters.
One such parameters is the interelectrode gap formed between the anode and the surface to be treated. While the interelectrode gap in a treatment apparatus is not as critical as the interelectrode gap in a foil forming apparatus (in that less power is required to treat an existing foil as compared to forming a new foil), a uniform interelectrode gap enables precise current density control along the path of the foil which, in conjunction with control of other parameters, produces more uniform surface treatment.
Other important parameters are the flow of the electrolytic fluid between the anode and the surface to be treated and the chemical composition of such fluid. In this respect, it is important to maintain a proper concentrations of plating ions in the vicinity of the foil to be paled (i.e. treated) and to replace ions which have been deposited onto the foil. Thus, the ability to control the electrolyte solution, i.e. its concentration and flow rate, also affects the efficiency of the treatment line.
Another parameter, which is generally related to both the electrolytic solution and the efficiency of the treating line is the temperature of the electrolytic solution. As is well known in the art, an electrode treating process creates considerable heat in the interelectrode gap, and such heat can cause current density fluctuations within the interelectrode gap. Forcing the electrolytic fluid through the interelectrode gap is one way of removing heat and generated gas therefrom. In this respect, by controlling the flow of electrolytic fluid through the gap, heat in the interelectrode gap can be reduced and the current density fluctuations can be minimized.
Heretofore, it has been known to provide an electrolytic cell by immersing a cathode surface in a large tank of electrolytic solution and by immersing an anode in the tank adjacent to the cathode surface. The large volume of electrolytic solution in such a tank provides a convenient heat sink to dissipate heat, and permits recirculation of the electrolytic fluid through a heat exchanger to maintain a selected temperature. A mixer may also be provided to maintain a selected chemical concentration in the electrolytic solution.
As will be appreciated, such arrangements generally require large amounts of electrolytic solution, but more importantly, generally, do not provide an accurate means of monitoring the actual solution concentration in the interelectrode gap or flow rate through the gap. Moreover, current density in the interelectrode gap may be affected by the electrolytic solution surrounding the anode and the concentration thereof.
The present invention relates to an apparatus for applying a surface treatment to metal foil, which apparatus provides greater control over the parameters which affect the efficient treatment of quality foil. The present invention allows greater control over the aforementioned treatment parameters by providing an anode which defines an interelectrode gap and at the same time generally functions as a tank to confine the electrolytic solution to such gap. According to the present invention, only the anode surface facing the foil to be treated is exposed to electrolytic fluid wherein an electrical potential exists only between the foil to be treated and the anode. By confining the electrolytic solution in a predetermined space of known dimension, the flow, temperature, and concentration of the electrolytic fluid can be monitored and controlled to optimize the treatment parameters and thus provide greater control over the current density in the interelectrode gap.